The commonly used cutting tools are solid end mills, which are made of cemented carbide from the tips of the edges to the shanks, and inserted tools, of which the bodies are made of steel and the edges are exchangeable carbide tips. Since solid end mills are formed integrally from the tip to the shank, they have high precision and are commonly used in finishing processing. Inserted tools have lower precision than solid end mills because of an insertion error due to tip-insertion process and the like, and are commonly used in rough processing. Also, inserted tools can be manufactured at low cost compared to solid end mills since only the tips of inserted tools are made of cemented carbide.
In recent years, exchangeable tip cutting tools in which a machining head having a cutting edge can be screwed to a holder are widely spreading. Exchangeable tip cutting tools can be manufactured at lower cost than solid end mills and generally have higher precision than inserted tools. Even if the machining head is an inserted tool, it is still useful because it can be modified into various types of tools by replacing only the machining head. These exchangeable tip cutting tools are designed so that the cutting load is applied in the tightening direction during cutting, and, therefore, the machining head does not unscrew during cutting. However, there are problems that fracture or breakage of the threaded section occurs when an enormous load is applied because of the increased cutting load due to abrasion of the tools. Also, even if there is no fracture nor breakage, there is still such a problem that the tightening torque increases during cutting and detaching the machining head becomes difficult.
To solve these problems, ideas to enhance the strength of threads by improving the shapes of threads and to make detaching the head easier are proposed. Also, not only for threads of the exchangeable tip cutting tools, but various proposals are made for commonly used threads as well.
In Patent Document 1, an exchangeable tip machining head in which the leading flank angle and the following flank angle of the male screw are optimized is described. It is described that use of a buttress thread having the following flank angle that is smaller than the leading flank angle makes it easier to be detached from a holder which is even expanded with cutting heat.
In Patent Document 2, in exchangeable tip cutting tool, it is described that, not only the shapes of the thread ridges are optimized to raise the strength, but also the head and the holder are fastened with tensile force which is larger than compressive force.
In Patent Document 3, a high strength bolt, which is excellent in fracture characteristic, is described. The high strength bolt has asymmetry shaped threads with different flank angles on the right and the left of the thread ridges, and the pressure flank angle is between 40° and 60°.
In Patent Document 4, a male thread member of which angle of inclination of a side surface, on which the load is applied at least when fastened, to the surface perpendicular to the central axis is 60° and/or more and is less than 90° is described.